In display devices such as liquid crystal display devices, color filters are widely used for the purposes of color image display, reflectance reduction, contrast adjustment, spectral characteristic control, and the like. A color filter is formed by arranging colored pixels in a matrix on a substrate. Methods for forming such colored pixels on a substrate include, for example, printing and photolithography.
FIG. 6 is an enlarged view of pixels of a color filter, and FIG. 7 is a cross-sectional view of the pixels, taken along a line X-X in FIG. 6.
The color filter shown in FIGS. 6 and 7 includes a substrate 50, a lattice-shaped black matrix 21 formed on the substrate 50, colored pixels 22, and a transparent conductive film 23. The black matrix 21 has a light-shielding property, defines the positions of the colored pixels 22 on the substrate 50, and makes the size of the colored pixels 22 uniform. In addition, when the color filter is used in a display device, the black matrix 21 blocks unnecessary light to achieve a high-contrast, even, and uniform image quality. The colored pixels 22 function as a filter for reproducing various colors.
A color filter is formed as follows. Firstly, a black photoresist is applied to the substrate 50, and exposed to light through a photomask and then developed, thereby forming a black matrix 21. Next, a color resist is applied to the substrate 50, and exposed to light through a photomask and then developed, thereby forming colored pixels 22. The process of forming colored pixels 22 is repeated until colored pixels 22 of all colors are formed on the substrate. Further, ITO (Indium Tin Oxide) is deposited by sputtering over the entire surface of the substrate 50 so as to cover the black matrix 21 and the colored pixels 22, thereby forming a transparent conductive film 23.
In mass production of the above-described color filter, it is general to form an array of a plurality of color filters on a single large substrate. For example, four color filters each having a diagonal of 17 inches can be formed on a glass substrate having a size of about 650 mm×850 mm.
As described above, in order to form a plurality of color filters on a single substrate, exposure has been popularly performed by using a photomask of approximately the same size as the substrate, on which a plurality of mask patterns corresponding to all the color filters are formed (for example, in the above-described example, a photomask on which four mask patterns corresponding to color filters each having a diagonal of 17 inches are formed). According to this method, patterns corresponding to all the mask patterns on the photomask are simultaneously formed on the substrate by a single exposure (so-called one-shot exposure).
However, the size of the photomask is increased with an increase in the size of the color filter. Thereby, the manufacturing cost of the photomask increases, and moreover, a problem of deflection of the photomask may occur due to its own weight at the time of exposure.
So, in order to resolve the problems of high cost and deflection due to an increase in the size of the photomask, an exposure method has been adopted, in which a plurality of exposures are performed by using a single photomask capable of simultaneously exposing a plurality of color filters, while changing the position of the photomask opposed to a substrate. For example, when the size of the substrate became about 730 mm×920 mm (the fourth generation), a single-axis step exposure method was adopted, in which exposure is repeated with the substrate being moved in steps along one direction with respect to a photomask. When the size of the glass substrate became about 1000 mm×1200 mm (the fifth generation), an XY (two-axis) step exposure method (step and repeat method) was adopted, in which exposure is repeated with the substrate being moved in steps along two directions with respect to a photomask.
FIG. 8 is a plan view illustrating an example of manufacturing of color filters by the XY step exposure method.
On a substrate 50, first to sixth exposure regions 1Ex to 6Ex are provided, in which six (two rows×three columns) color filters are to be exposed. The substrate 50 is placed on an exposure stage 60, and is freely movable in the X and Y directions.
Firstly, exposure is performed with a photomask PM being overlapped with the first exposure region 1Ex to form a mask pattern of the photomask PM in the first exposure region 1Ex. Thereafter, the substrate 50 is moved by a distance Py in the positive direction of the Y axis to overlap the photomask PM with the second exposure region 2Ex, and a pattern of the photomask PM is formed in the second exposure region 2Ex. Next, the substrate 50 is moved by a distance Px in the positive direction of the X axis to overlap the photomask PM with the third exposure region 3Ex, and a pattern of the photomask PM is formed in the third exposure region 3Ex. Thereafter, in a similar manner to above, exposure is repeated with the substrate 50 being moved in the X direction or the Y direction, thereby forming patterns in the fourth to sixth exposure regions 4Ex to 6Ex.
The use of the XY 2-axis step exposure method resolves the problem of an increase in manufacturing cost due to an increase in the size of the photomask, and the problem of deflection of the photomask due to its own weight. However, if the size of the substrate is further increased (for example, about 1500 mm×1800 mm (the sixth generation) or about 2100 mm×2400 mm (the eighth generation)), the color filters themselves formed on the substrate are also increased in size, which eventually causes an increase in the size of the photomask. As a result, the problems of high cost and deflection of the photomask occur again.
So, an exposure method is attempted, in which exposure is continuously performed by using a photomask smaller than a single color filter, while transferring a substrate.
FIG. 9 is a plan view illustrating a slit exposure method. FIG. 10 is a cross-sectional view taken along a line X-X in FIG. 9. FIG. 11 is a partially enlarged view of a mask pattern of a photomask shown in FIG. 9. FIG. 12 is a partially enlarged view of stripe patterns exposed by the slit exposure method. In FIG. 10, part (a) shows a state where exposure of a first exposure region is started, and part (b) shows a state where exposure of the first exposure region is completed.
As shown in FIGS. 9 and 10, in the slit exposure method, a photomask PM2, which is smaller in size than a first exposure region 1Ex of a substrate 50 placed on an exposure stage 60, is disposed between the substrate 50 and a light source (not shown). The exposure stage 60 is movable at a constant speed in the horizontal direction of the figure, and further, is movable in steps in the vertical direction of the figure along the Y axis. As shown in FIG. 11, the photomask PM2 has a slit S for exposing a portion of a pattern formed in the first exposure region 1Ex. In the longitudinal direction Ls of the slit S, a plurality of openings 51 are aligned at predetermined intervals Pi. The width and length of each opening 51 are Wi and Li, respectively.
When exposing the first exposure region 1Ex, as shown in FIGS. 9 and 10(a), the photomask PM2 is placed on the left end of the first exposure region 1Ex. Then, while irradiating the photomask PM2 with a light beam from the light source, the substrate 50 is continuously transferred leftward in FIG. 6 along the X axis, until reaching the state shown in FIG. 10(b). As a result, as shown in FIG. 12, stripe patterns each having a width Wi and an interval Pi are formed, on the substrate 50, extending in the substrate transfer direction (the horizontal direction of FIG. 9).
After the exposure of the first exposure region, the exposure stage 60 is moved by a distance Py in the positive direction of the Y axis in FIG. 9 to align the photomask PM2 to an exposure start position in the second exposure region. Then, stripe patterns are formed in the second exposure region by performing continuous exposure similar to that performed on the first exposure region.
Thus, the slit exposure method realizes large-area exposure as well as a size reduction of the photomask.
FIG. 13 is a partially enlarged view of a color filter manufactured by the slit exposure method.
In the color filter shown in FIG. 13, stripe colored patterns extending in the X direction are formed on a glass substrate on which a lattice-shaped black matrix 21 is formed, thereby forming red colored pixels 22R, green colored pixels 22G and blue colored pixels 22B. In the Y axis direction, a set of red, green, and blue colored pixel lines is repeatedly formed at a pitch Pi.
Also in the case of manufacturing color filters using the slit exposure method, it is general to form a plurality of color filters on a single substrate to realize mass production.
FIG. 14 is a plan view illustrating an example of manufacturing color filters by the slit exposure method. FIG. 15 is a cross-sectional view illustrating a method of forming an area of a substrate shown in FIG. 14, taken along a line X-X in FIG. 14. In FIG. 14, shaded portions at both sides of each display region represent regions where a front end and a rear end of a stripe-shaped colored pattern are located, respectively. A photomask shown in FIG. 15 is identical to that shown in FIG. 11, and has a slit including a plurality of openings and shielding parts.
Six (two rows×three columns) regions 54A and regions 54B surrounding the regions 54A are formed on the substrate 50. A stripe pattern of a colored layer is formed on each of the regions 54A. On the other hand, a stripe pattern of a colored layer is not formed on the regions 54B. The substrate 50 is placed on an exposure stage 60, and is freely transferred in the X and Y directions. A photomask PM2 is fixed in a position above the substrate 50, which position is irradiated with a light beam E from a light source. Further, a blind shutter BS is provided between the photomask and the light source (not shown) so as to be movable in the X-axis direction in FIG. 15.
The blind shutter BS blocks the light beam from the light source. The blind shutter BS includes an upper shielding plate and a lower shielding plate. The upper shielding plate and the lower shielding plate are individually and freely movable in the X-axis direction by means of a movement mechanism (not shown).
When performing exposure on the region 54A, the slit (not shown) of the photomask PM2 is irradiated with the light beam E from the light source, with the upper shielding plate and the lower shielding plate of the blind shutter BS being opened in the horizontal direction of FIG. 15, while continuously transferring the substrate 50 in the direction indicated by an outlined arrow. Thereby, a stripe pattern is formed on the region 54A.
After the exposure of the region 54A is completed, the upper shielding plate and the lower shielding plate of the blind shutter BS are closed to shield the photomask PM2 from the light beam. Thereby, a region 54B having no stripe pattern is formed.
Thereafter, exposure and shielding according to open and close of the blind shutter BS are repeated while transferring the substrate 50 in the outlined arrow direction to form stripe patterns on the three regions 54A arrayed in the X axis direction shown in FIG. 14.
When performing shielding by means of the blind shutter BS, the light beam E is diffracted at edges c and d of the blind shutter BS. Thereby, the amount of light irradiation on the resist becomes insufficient in regions G shown in FIG. 15.
FIG. 16 is a cross-sectional view of an end portion of a stripe pattern. The stripe pattern shown in FIG. 16 is obtained by developing a substrate exposed by the slit exposure method described with reference to FIGS. 14 and 15.
The stripe pattern 22 shown in FIG. 16 is formed so as to have a thickness (H1). However, due to the above-described insufficient irradiation, the thickness of the stripe pattern 22′ in the region G (corresponding to the left-side region G in FIG. 15) is relatively thin.
The thickness of the stripe pattern 22′ is gradually reduced toward an end g. Specifically, the thickness of the stripe pattern 22′ is gradually reduced in order of (H1), (H1-ΔH3), (H1-ΔH2) from the left to the right of FIG. 16 (ΔH2>ΔH3).
The length of the region G (in the horizontal direction of FIG. 16) is 500 μm at maximum, and varies within a range of 300 μm to 500 μm. Such a reduction in thickness is caused by the type of the photoresist, the interval (gap) between the photomask and the substrate during proximity exposure, or the moving speed of the substrate. If such a thickness-reduced portion is positioned in the display region, a reduction in display quality may occur.
In the color filters, columnar spacers are formed in addition to the above-described colored layers. Hereinafter, the columnar spacers will be described with reference to FIGS. 17 to 19.
FIG. 17 shows an example of a color filter having columnar spacers. FIG. 18 is a cross-sectional view of the color filter shown in FIG. 17, taken along a line X-X in FIG. 17.
In the color filter shown in FIGS. 17 and 18, columnar spacers Cs are formed on a transparent electrode 23 above a black matrix 21 disposed on the substrate 50. Generally, the columnar spacers Cs are also formed in regions other than display sections. Hereinafter, the columnar spacers formed in regions other than the display sections are referred to as “dummy columnar spacers”. The dummy columnar spacers maintain a uniform interval between the substrate other than the display sections and a counter substrate, and thus the interval between the substrate and the counter substrate can be kept uniform in the display regions.
FIG. 19 is a diagram illustrating an example in which a plurality of color filters are formed.
As described above, in the case where six (two lows×three columns) color filters are formed on a single substrate 50, the substrate 50 is divided into: display sections A in which colored pixels of the color filters are formed; frame sections B surrounding the display sections A; interplanar regions F between the respective display sections A; and a peripheral section D of the substrate 50. A region enclosed in a broken line C (a boundary between the frame section B and the peripheral section D) corresponds to the finished size of a color filter. The columnar spacers are formed in the display sections A, and the dummy columnar spacers are formed in the frame sections B, the interplanar regions F, and the peripheral section D.
FIG. 20 is a partial sectional view of a color filter manufactured by the step exposure method.
In a display section A, a black matrix 21, a colored pixel 22, and an ITO film 23 are disposed. A plurality of columnar spacers Cs are formed on the ITO film 23. On the other hand, a plurality of dummy columnar spacers D-Cs are formed at a pitch (Pi-2) in the frame section B, the interplanar region F, and the peripheral section D.
The colored pixel 22 is formed with a uniform thickness throughout, including a right end portion f. As shown by a broken line H, the columnar spacers Cs and the dummy columnar spacers D-Cs are formed so as to have uniform height.
FIG. 21 is a partial sectional view of the color filter formed by the slit exposure method.
In the color filter shown in FIG. 21, the stripe pattern 22′ (a portion of the stripe pattern 22 in the region G), whose thickness is relatively thin due to the influence of the above-described diffraction, is formed in the display section A. If this color filter is incorporated in a display device, a reduction in display quality may occur. In order to avoid this problem, there is proposed a technique of adjusting the position of the end portion of the stripe pattern, as shown in FIG. 22.
FIG. 22 is a cross-sectional view illustrating another example of a color filter formed by the slit exposure method.
In the color filter shown in FIG. 22, the stripe pattern 22′ (a portion of the stripe pattern 22 in the region G), whose thickness is relatively thin, is formed in the frame section B. In this structure, the thickness (H1) of the stripe pattern 22 in the display section A is made uniform to avoid a reduction in display quality.